Of various solar cells broadly classified into silicon solar cells, thin-film solar cells, and compound solar cells, thin-film solar cells have been commercialized as optical devices using thin-film technology because its manufacturing process is simple and achieves low energy. A chalcopyrite type thin-film solar cell belongs to thin-film type and comprises a CIGS layer made of a chalcopyrite compound (Cu(In+Ga)Se2:CIGS) as a p-type light absorbing layer. It is known that the light absorbing layer made of such a compound enables a solar cell to have high photoelectric conversion efficiency particularly when the light absorbing layer is combined with a glass substrate containing an alkali metal such as soda lime glass. Research and development have been conducted for mass-producing such a thin-film solar cell because of its high radiation resistance as well as high reliability obtained by remarkably reducing photo deterioration (secular change) arising from a lattice defect and the intrusion of an impurity, photosensitivity obtained in a wide light absorbing wavelength region including a long wavelength band, and a high optical absorption coefficient.
FIG. 1 shows the layer structure of a typical thin-film solar cell comprising a CIGS layer as a light absorbing layer. The solar cell has a multilayer structure 7 which comprises, on a soda lime glass (SLG) substrate 1, an underside electrode layer 2 acting as a positive electrode and including an Mo metal layer, a Na dipping layer 3 for preventing uneven Na distribution caused by the SLG substrate 1, the above-mentioned CIGS light absorbing layer 4, an n-type buffer layer 5, and an outermost transparent electrode layer 6 acting as a negative electrode.
When light such as sunlight is incident on an upper light-receiving part of the solar cell, excitation caused by the light having energy not lower than a band gap generates a pair of an electron and a positive hole around the p-n junction of the multilayer structure 7. The excided electron and positive hole are diffused to the p-n junction, and the internal electric field of the junction separates the electron and the positive hole to an n-region and a p-region, respectively. As a result, the n-region is negatively charged and the p-region is positively charged, causing a potential difference between electrodes 8 and 9 provided in the respective regions. The potential difference acts as electromotive force and provides photocurrent for the electrodes connected via a conductor. This is the principle of the solar cell.
FIG. 2 is a process drawing showing a process for producing the chalcopyrite type thin-film solar cell having the multilayer structure 7 shown in FIG. 1.
In the production of the solar cell, first, the Mo electrode layer is formed on a clean glass substrate made of SLG or the like according to sputtering technique using a metallic Mo target (Mo electrode layer forming step: FIG. 2(a)).
Then, the substrate where the Mo electrode layer is formed is divided to a desired size by laser cutting (first scribing step: FIG. 2(b)).
After the substrate is cleaned with water and so on to remove shavings or the like therefrom, the substrate is dipped into a diluted solution of a sodium containing compound such as sodium chloride (Na dipping layer deposition step: FIG. 2(c)). Thereafter, a two-layer structure of an In layer and a Cu—Ga layer is formed according to sputtering deposition using a metallic In target and a Cu—Ga alloy target (the step of forming the precursor of the light absorbing layer: FIG. 2(d)).
For example, in a conventional method of obtaining the CIGS light absorbing layer, as shown in FIG. 2(e), the substrate on which the precursor of the lower In layer and the upper Cu—Ga layer is superimposed is accommodated in an annealing chamber and preheated at 100° C. for ten minutes. After the preheating, hydrogen selenide (H2Se) gas is introduced through a gas inlet tube inserted into the annealing chamber, and the interior of the chamber is heated so as to range from 500° C. to 520° C. while the gas is passed through the chamber. Such annealing transforms the precursor having a laminated structure of the In layer and the Cu—Ga layer into a signal CIGS layer. At this point, the Na dipping layer is diffused in the light absorbing layer and disappears therein. After the heat treatment, the hydrogen selenide gas acting as reactant gas is replaced with purge gas such as Ar gas, and then the purge gas is cooled (Japanese Laid-Open No. 2003-282908).
After the substrate with the CIGS layer is removed from the annealing chamber, the buffer layer is formed on the substrate using an n-type semiconductor material including CdS, ZnO, and InS according to chemical bath deposition or sputtering technique shown in FIG. 2(f).
Further, on the substrate where the buffer layer is formed, cutting is performed using laser irradiation and a metal needle (second scribing step: FIG. 2(g)).
Thereafter, the outermost transparent conductive layer including a ZnOAl layer is formed according to sputtering technique using a ZnO—Al alloy target (FIG. 2(h)).
And then, on the substrate where the transparent conductive layer is formed, cutting is performed again using laser irradiation and a metal needle (third scribing step: FIG. 2(i)).
The above-mentioned thin-film solar cell having the laminated structure is obtained as a single cell made uniform in size by cutting. A final product has a flat laminated structure where these cells are connected in series.
Incidentally, in the case of the conventional thin-film solar cell including the CIGS light absorbing layer, in the step of transforming the precursor having the laminated structure of the In layer and the Cu—Ga layer into the single CIGS layer, film components of Cu, In, Ga and Se are locally present and unevenly distributed in the film structure of the formed CIGS light absorbing layer.
Particularly when the Cu—Ga layer and the In layer are superimposed in this order, alloying caused by solid layer diffusion tends to occur on an interface between the Cu—Ga layer and the In layer. Three components other than Se tend to be alloyed or the In layer tends to be deposited on the Cu—Ga layer, resulting in difficulty in transformation into the desired single CIGS layer. Moreover, when CIGS is crystallized by annealing, the film structure is reconstructed to increase a filling rate. The reconstruction involves an increase in film thickness. When the film increases in thickness, the constituent components are unevenly distributed along the thickness direction in the obtained single CIGS layer due to a difference in solid layer diffusion coefficient between the constituent components.
For example, it is needless to say that Ga components distributed with relatively high density on an electrode layer side are preferable in view of energy band and the unevenly distributed constituent components do not always cause a problem. However, it is known that adhesion is poor on the interface of the Mo electrode layer and metal Ga. When Ga components are unevenly distributed on the electrode layer side, the Ga components cause, in many cases, segregation on the interface of a high-density side. The segregation of the Ga components results in poor adhesion between the light absorbing layer and the electrode layer, so that the obtained thin-film solar cell has a structural problem of internal exfoliation. Patent Document 1: Japanese Patent Laid-Open No. 2003-282908